1. Field of the Invention
The present invention relates to a photoelectric reading apparatus for reading an image of an original using a light source that projects light onto the original. The light source can be defined by three lamps of different colors, e.g., red, green and blue, for reading the image in colors or a single lamp for reading the image in monochrome.
2. Description of the Prior Art
The reading apparatus is used, for example, in a facsimile machine or copying machine in which the original is scanned across the reading apparatus.
In a prior art reading apparatus, the reading apparatus includes a light source, such as one or a plurality of fluorescent lamps, for producing a ribbon of light, a scanner for scanning the original across the ribbon of light, a single array of CCD sensor, and an optical system for directing the ribbon of light reflected on the original onto the CCD sensor.
The prior art reading apparatuses of the above described type have a problem in poor image reading due to the temperature characteristics of the fluorescent lamp, as explained below.
The fluorescent lamp has such temperature characteristics that: (i) the intensity of light generated from the fluorescent lamp alters depending on the temperature of the atmosphere surrounding the fluorescent lamp; and (ii) the intensity of light generated from the fluorescent lamp alters depending on the temperature of the inner wall of the fluorescent lamp tube. Thus, a problem arises such that the amount of light impinging on the original is insufficient or gradually changes during the image reading. This results in a low output signal or gradually changing output signal from the CCD sensor.
Referring to FIG. 1, a relationship between the intensity of light generated from the fluorescent lamp and the temperature of the lamp tube wall is shown. As shown in FIG. 1, the light intensity is at a peak point when the temperature of the wall is about 40.degree. C. to 50.degree. C., and the light intensity gradually decreases both when the temperature decreases or when the temperature increases.
FIG. 2a shows a relationship between the CCD output power and different colors obtained under 10.degree. C. Similarly, FIG. 2b shows the same relationship, but obtained under 40.degree. C. In both graphs, the shaded areas show the amount of noise signal contained in the output power. When these two graphs are compared, the SN ratio of output power under 10.degree. C. is much greater than that under 40.degree. C., and therefore, the image reproduced using the CCD sensor output under 10.degree. C. will be very poor, when it is compared with the image reproduced using the CCD sensor output under 40.degree. C. Therefore, the image reading should be carried out when the light intensity of the emitted light from the fluorescent lamp is at or around its peak point.
Referring to FIG. 3, a relationship between the CCD output power and the time in which the fluorescent lamp is turned on, obtained during the color original reading and under the condition that the atmospheric temperature is 20.degree. C., is shown. In FIG. 3, the gradual increase of the CCD output power relative to the time is due to the gradual temperature increase of the tube wall of the fluorescent lamp.
Generally, as shown in FIG. 4, when reading the original in colors, three fluorescent lamps, such as red, green and blue, are lit sequentially by three control signals R, G and B. After three lamps are lit, a non-lighting period is provided for data transfer. Thus, each control signal has a duty cycle of 25%.
As understood from FIG. 3, even with the same original, the output data from the CCD sensor obtained immediately after the turning on of the light source differs from that obtained a few minutes after turning on of the light source. Such a difference results not only in the low contrast image but also in an unbalanced color image.